Touchscreens are well-established computer input devices. Uses of touchscreens include point-of-sale applications like cash registers at fast-food restaurants, point-of-information applications such as department store information kiosks, and ticketing applications such as airline-ticket kiosks. As touchscreen technologies mature, the range of applications increases. To the extent that touchscreen technology can economically be made weather resistant and vandal resistant, the market for touchscreens will expand into outdoor and semi-outdoor applications.
Commercially available touchscreens utilize a variety of different touch detection mechanisms. These detection mechanisms include interruption of infrared (IR) optical beams; electrical contact to a voltage gradient on a transparent resistive coating via flexing of a flexible coversheet; absorption of ultrasonic acoustic waves propagating along the touchscreen surface; capacitive shunting of an oscillating current through either a very thin or a thick dielectric layer to a user's finger and then through the user's body to ground; and detection of a touch on a touchscreen via force sensors supporting the corners of the screen.
To date, the touchscreen market has been dominated by resistive, acoustic, and thin-dielectric capacitive touchscreens. For a variety of reasons, however, it is doubtful that any of these can fully meet the requirements imposed by an outdoor application. For example, the plastic coversheet used in a resistive touchscreen is easily vandalized by sharp objects (e.g., knives) or by burning (e.g. cigarettes). Similarly, shallow scratches on the surface of a thin-dielectric capacitive touchscreen can lead to unacceptable distortion in the measured touch coordinates thus making the touch detection mechanism susceptible to vandalism as well. Acoustic touchscreens are affected by water contaminants and therefore are typically not selected for any application in which the touchscreen may be directly or even indirectly exposed to rain (e.g., water dripping from wet clothes or an umbrella).
One type of touch detection mechanism that appears to be well suited for outdoor applications is based on a thick-dielectric capacitive touchscreen. Such systems are often referred to as projective capacitive touchscreens since the detection mechanism involves projecting electric fields through a thick dielectric layer. This type of touchscreen is fundamentally insensitive to moisture, e.g., rain drops, on the touch surface. Additionally, the material comprising the exterior touch surface plays no fundamental role in the touch detection mechanism, thus providing design flexibility. For example, a replaceable, low-cost layer can be used as the exterior touch surface.
In a typical projective capacitive sensor, three transparent substrates (e.g., glass) are laminated together, each substrate having a patterned transparent resistive coating. The patterned resistive coatings are fabricated from a material such as ITO or ATO. Silver frit traces are typically used to couple the patterned coatings to the detection electronics. In one configuration, the underside of the top substrate layer has horizontal Y-measuring electrodes while the top surface of the middle substrate glass has vertical X-measuring electrodes. The upper Y-measuring electrodes can be patterned in such a way as to minimize shielding of the underlying X-electrodes. The top surface of the bottom substrate layer contains a back guard electrode to isolate the sense electrodes from the electronic environment behind the touchscreen (i.e., display device). Thus in this configuration the X- and Y-electrodes are contained within separate planes.
In this type of projective capacitive sensor, the stray capacitive cross-coupling between the X- and Y-measuring electrodes is inversely proportional to the distance between the X- and Y-electrode planes. Therefore reducing the gap between the X- and Y-electrode planes increases the capacitive cross-coupling, possibly leading to an increase in the demands placed on the associated electronics.
PCT application WO 95/27334 and U.S. Pat. No. 5,844,506 disclose another type of projective capacitive touchscreen utilizing fine wires of between 10 and 25 micrometers thick as the electrodes. A variety of techniques are disclosed for electrically isolating the electrodes. For example in one configuration the two sets of electrodes, i.e., the X- and Y-electrodes, are applied to opposite faces of a thin dielectric film. In another configuration the two sets of electrodes are applied to the same side of the dielectric substrate or film. Methods of electrically isolating the sets of electrodes include the deposition of an insulating layer between the two electrode sets, the insulating layer either being continuous over the entire touchscreen surface or applied locally to the intersections of the two electrode sets. In at least one of the disclosed touchscreen systems, the dielectric support substrate or film as well as the two sets of electrodes are encapsulated in a dielectric laminate, thereby eliminating the influence of moisture as well as providing a constant dielectric environment in the immediate proximity of the electrodes.
U.S. Pat. No. 4,954,823, by the same inventor as PCT Application No. WO 95/27334, discloses a touchscreen control system for use with projective capacitive touchscreens such as those disclosed in the PCT application. In the disclosed control system the electronics measure changes in sense-electrode capacitances due to touches via shifts in the frequency of a RC-time-constant controlled oscillator. A similar technique utilizing a RC-time-constant controlled oscillator for measuring the capacitance changes in a touch sensor is disclosed in U.S. Pat. No. 4,103,252.
U.S. Pat. No. 5,650,597 discloses a projective capacitive touchscreen in which both the X- and Y-electrodes are formed in a single plane, the electrodes arranged as an array of horizontal sensor bars. A control circuit provides an excitation signal to the electrodes and receives a differential signal from the electrodes, the differential sensing signal providing touch position. In order to determine horizontal position, the excitation signal is provided to one side of the array and the sensing signal is received on the other side of the array. A similar array of unidirectional electrodes and a method of obtaining both X- and Y-coordinate information therefrom is disclosed in U.S. Pat. No. 4,778,951. Projective capacitive touchscreen designs such as those disclosed in U.S. Pat. Nos. 4,778,951 and 5,650,597 are dependent upon manufacturing processes that produce transparent conductive coatings of uniform resistivity that are substantially free of defects that can break the electrical continuity of the coating.
PCT Application No. WO 96/15464 discloses a controller for use with a variety of touchscreen sensor array types. The signals from the array are processed both in the analog and digital domains, thus achieving reliable touch detection. In one of the disclosed signal processing techniques, the information from several sensor areas is interpolated to achieve fine positional resolution. The disclosed sensor arrays include a single surface array of pads that is substantially symmetrical, an asymmetrical array of pads (e.g., "Backgammon grid"), and dual orthogonal arrays arranged on different substrate layers.
A projective capacitive touchscreen using two sets of sensing electrodes applied to a single substrate surface to accurately determine the X- and Y-coordinates of a touch is desired. The present invention provides such a system.